1. Field of the Invention
The present invention relates to a clock generator which generates a clock signal of which oscillation frequency is diffused with spectrum to reduce electromagnetic wave radiation due to high-speed oscillation operation.
2. Description of Related Art
As higher performance in electric appliances has conventionally been advanced, operation of appliance has been sped up further more and higher integration design of semiconductor devices for those appliances and theirs packaging substrates has been developed. An oscillation period of a clock signal and the like is shortened to meet with the level same as wiring length of signal wirings on a packaging substrates and its semiconductor device, which results in that signal wiring functions as an antenna and electromagnetic wave radiation, unnecessary radiation, increases.
Furthermore, portable electric appliances have become popular for recent years and suppression of electromagnetic waves radiation is demanded for those portable electric appliances. Since miniaturization and weight saving are required for portable electric appliances, conventional countermeasure such as improvement of circuit arrangement design, insertion of shielding material against electromagnetic wave and the like are restricted before such necessity.
As countermeasure, there has been suggested so-called spectrum diffusion technology for changing oscillation frequency of a clock signal. As an example for such case that a clock signal to which spectrum diffusion is applied is outputted in a PLL circuit, JP Laid-open Patent publication No. 2000-101424 discloses related technology.
First technology is such that an integrated clock signal is obtained by integrating a to-be-inputted clock signal with an integrator, and in a limiter, an integrated clock signal is modulated by a frequency control signal of which voltage level changes with frequency lower than that of a clock signal, whereby there is obtained an output clock signal of which cycle continuously changes. To be more specific, voltage level of an integrated clock signal and that of a frequency control signal are compared, whereby cycle of an integrated clock signal is modulated with changed frequency responsive to voltage level of a frequency control signal and an output clock is obtained. Oscillation frequency is changed by inputting an output clock signal of which frequency changes to a PLL circuit.
Second technology is such that there is provided a direct current amplifier between a low pass filter (LPF) and a voltage control oscillator (VCO) and a frequency control signal of which voltage level changes is inputted to the direct current amplifier.
Third technology is such that, in a charge pump, bias current which generates in response to a frequency control signal is added to charge current which generates in response to an output signal from a phase comparator and voltage level of a capacitor charged/discharged depending on current is controlled, whereby voltage level of a control signal to be inputted to a voltage control oscillator (VCO) is controlled so as to control oscillation frequency.